1. Field
The present invention relates generally to magnetic memory and more specifically to magnetoresistive memory elements.
2. Related Art
In magnetoresistive random access memory (MRAM), data is stored by using an electric current to create a magnetic field for switching a magnetic direction of a ferromagnetic layer of a multilayer memory element (also known as a bit). An MRAM is described as a type of nonvolatile memory because a logical state of a memory element will persist even when power is removed from the memory device. Although there are other types of nonvolatile memories, MRAM allows rapid read and write sequences. And unlike dynamic random access memory (DRAM), MRAM does not require a constant flow of current to retain its data—thus potentially consuming less power.
Traditional MRAM elements have a rectangular shape with tapered bit ends and a linear magnetization. One type of MRAM element is a giant magnetoresistive (GMR) device that may be manufactured as a “spin valve” or “pseudo-spin valve,” for instance. GMR devices, including spin valves and pseudo spin valves, can be used as data storage elements in magnetic random access memory (MRAM) devices. In this regard, exemplary MRAM applications of GMR devices are described in U.S. Pat. Nos. 6,147,922; 6,175,525; 6,178,111; and 6,493,258, all of which are incorporated herein by reference.
In the rectangular elements, magnetic poles generated at bit-ends can form demagnetizing fields within the bit and stray fields outside the bit. The demagnetizing fields can create complex magnetic domains within the bit that are detrimental to switching thresholds and data retention. The stray fields can interfere with adjacent bits. Tapering the bit ends has been useful for spreading and weakening the magnetic poles; however, problems have not been eliminated.
Ring-shaped magnetoresistive elements have been proposed as a solution to the problem of demagnetizing and stray fields. The circular magnetization mode eliminates magnetic poles and the resulting demagnetizing and stray fields. However, implementations of ring-shaped elements have been limited to low impedance giant magnetoresistive (GMR) devices and require that a high current be passed vertically (perpendicular to the plane) through the element to generate circumferential magnetic field that writes the bit. Because of the direction of current, the ring-shaped elements are known as current perpendicular to the plane (CPP) ring-shaped devices.